In the manufacture of integrated circuits, the various layers which form the integrated circuit are patterned using photolithography. In the photolithography process a layer of photosensitive material called photoresist is first formed on the surface of the layer to be patterned. A pattern is then transferred to the photoresist from a photomask. The photomask typically comprises a glass slide on which an opaque pattern has been formed. By positioning the photomask over the surface of the photoresist and exposing the photomask to UV radiation, the opaque pattern on the photomask will be transferred to the photoresist. Exposing the photoresist to light will change the molecular structure of the material and allow certain regions of the photoresist to be removed. If positive photoresist is used then the regions exposed to the UV radiation will be removed. If negative photoresist is used then the regions of the photoresist which were not exposed to the UV radiation will be removed. Removal of these various regions of the photoresist allows the pattern in the photoresist to be transferred to the underlying layers.
The minimum pattern linewidth which can be formed in the photoresist is dependent on a number of factors including the wavelength of the radiation used to expose the photoresist as well as the formation of standing waves in the photoresist layer during the exposure. For a given radiation wavelength the presence of standing waves in the photoresist layer will severely limit the minimum pattern linewidth which can be formed. Standing waves are formed in the photoresist by reflection of the incident radiation from the upper and lower surfaces of the photoresist layer. To reduce the standing waves formed in the photoresist layer an anti-reflective coating (ARC) layer is often placed between the layer to be patterned and the photoresist layer. Such an arrangement is shown in FIG. 1(a). An anti-reflective coating (ARC) layer 20 is formed on the layer to be patterned 10. In semiconductor processing the layer 10 can comprise silicon, polycrystalline silicon, dielectric layers such as silicon oxide and silicon, metal, or any material which may be used in fabricating the integrated circuit. A number of materials have been used to form the ARC layer including polymers and silicon nitride. For the very narrow line widths required in modern integrated circuits it is preferable to use silicon nitride to form the ARC layer 20. For the case where the ARC layer 20 comprises silicon nitride, the pattern is first transferred to the photoresist film 30 using the method described above. The patterned photoresist film is then used as a masking layer to transfer the pattern to the ARC layer 20 and the layer to be patterned 10. For the case of patterned lines, an example of a patterned photoresist layer is shown in FIG. 1(b) where the patterned photoresist lines 35 are shown. The patterned photoresist lines 31 shown in FIG. 1(b) are flared at the base of the lines. This flaring will limit the shape of the profiles of the patterned transferred to the ARC layer 20 and the layer to be patterned 10 and will also affect the minimum line width and line width uniformity that can be obtained from a particular technology. In the case of silicon nitride ARC layers the flaring is due to the poisoning of the photoresist film 30 by the silicon nitride ARC layer 20. In order to reduce and/or eliminate the flaring of the photoresist lines 35 the photoresist poisoning effect has to be reduced and/or eliminated. There is therefore a great need for a method that reduces and/or eliminates photoresist poisoning when using silicon nitride and other related ARC layers.